Extensive research over the past few years has been focused on the synthesis and characterization of microporous materials with high internal surface areas. Metal-Organic Frameworks (MOFs), a crystalline subset of these materials, have shown promise in a wide range of applications from gas storage, chemical separations, chemical sensing, and catalysis, to ion exchange, light harvesting, and drug delivery. High internal surface area is one of the foremost attributes of MOFs and has been shown to be highly desirable in many potential applications involving catalysis or storage. Also relevant is the sorption-based storage of technologically important gases at temperatures above their respective critical temperatures. For example, at T>191 K methane cannot form methane/methane multi-layers. Thermodynamically excess adsorption under these conditions, therefore, can be achieved only via direct contact between individual methane molecules and the sorbent surface.
Additionally fueling interest in MOFs is their extraordinary compositional and structural variety (e.g., ca. 10,000 experimentally known MOFs versus fewer than 300 zeolites) and the fact that many display permanent porosity, ultra-low densities, and well-defined pores and channels. Further, the crystallinity of MOFs allows for their unambiguous structural characterization by X-ray diffraction, greatly simplifying efforts to use computational modeling to predict or explain their unusual or unique physical properties.
One of the first breakthroughs in obtaining MOFs with permanent microporosity came in 1998 from Li et al., who described a material having a Langmuir surface area of 310 m2/g. Increases in reported surface areas for MOFs followed for the next several years, with values reaching 3,800 m2/g in 2005 and 5,200 m2/g in 2009. Among the reported high-area materials are MOF-5 (especially in anhydrous form), MOF-177, MIL-101, UMCM-1, and UMCM-2 (See Table 1 discussed in more detail below).
Efforts to achieve even higher surface areas stalled, not primarily because of difficulty in synthesizing new candidate materials, but because of the progressively greater tendency of these materials to collapse upon removal of solvent. However, a MOF activation method based on super-critical carbon dioxide (discussed in more detail below) has enabled difficult-to-activate, large-cavity MOFs to be evacuated without framework collapse or channel blockage. Based on this method, two MOFs with experimentally accessible BET surface areas slightly above 6,000 m2/g have been reported: MOF-210 and NU-100 (NU-100 is also known as PCN-610).
Some researchers believe that the reported record-high surface areas for NU-100 and MOF-210 are close to the ultimate [experimental] limit for solid materials. This belief stems from: a) simulations showing that the upper theoretical limit for MOF surface areas is about 10,500 cm2/g when linkers are constructed from repeating phenyl groups, and b) anticipated practical problems, such as poor solubility, low synthetic yields, and cumbersome purification protocols, for candidate linkers featuring very large numbers of phenyl repeat units.
Attempts have also been made previously to calculate the highest possible surface area for a porous material. Chae at al. describe a useful conceptual basis for a strategy to achieve high-surface-area ordered materials. By progressively excising smaller fragments from an infinite graphene sheet and calculating Connolly surface areas of the remaining framework, it is found that exposing all latent edges to give isolated six-membered rings would yield a surface area of 7,745 m2/g. The exposed six-membered rings are essentially benzene molecules without hydrogens, whose inclusion would have given an even higher surface area. By putting this strategy into practice, MOF-177 is synthesized from Zn4O clusters and 1,3,5-benzenetribenzoate (BTB) organic linkers, which shows a record-breaking surface area for that time (4,750 cm2/g). Subsequently, it was realized that for sorption applications, molecule-accessible surface areas are physically more meaningful than Connolly surface areas. Additionally, Snurr and co-workers showed that, subject to well defined “consistency criteria”, experimental BET surface areas for fully evacuated MOFs (but not Langmuir or Connolly surface areas) correspond closely to molecule-accessible surface areas.
In a related approach, Schnobrich et al. constructed a series of structures (in silico) by incrementally adding benzenes to the linker of MOF-5 (1,4-terephthalic acid). This study revealed that a MOF-5 analogue with an infinite number of benzenes in its linker would give an N2-accessible surface area of 10,436 m2/g, which is very close to the maximum attainable surface area (10,577 m2/g) for structures derived from benzene rings regardless of their topology. These studies have, until now, largely defined surface-area targets for MOF materials for both experimental and theoretical investigations, and the use of benzene chains of different forms and lengths has become a common way of synthesizing materials with high surface areas.